Achieving acoustic silence is an important concern in marine vessels. For passenger ships the purpose is to improve passenger comfort, whereas for scientific ships or naval vessels, silence is an operating constraint.
It is therefore appropriate to reduce the acoustic vibration generated by water being compressed against the hull because of propeller rotation. This corresponds to the case where the ship is driven by at least two drive shafts each provided with a propeller.
It is thus known to fit each of the two shafts of a ship with an angular position encoder and to use these means to regulate the speed of rotation of the shafts so that they present an angular difference which is set to a constant reference value. This reference is the result of mechanical considerations concerning the interactions between the propellers, the water, and the hull, and these considerations are subject to a certain amount of inaccuracy. In addition, the interactions depend closely on the operating conditions of the ship. Amongst these conditions, mention can be made in particular of the power developed on a drive shaft, its speed of rotation, the burden of the ship, the speed at which it is advancing, and the temperature and the salinity of the water.
As a result, calculating such a reference value is very complex. Furthermore, the fact of adopting a reference value that is constant leads necessarily to a compromise, in other words the reference value is not always optimized for the operating conditions of the ship in any given situation.